The present invention relates to signal demodulators, and more particularly to selectable data rate, digital PSK demodulators.
PSK modulation techniques are widely used in digital signaling over RF communications channels. In digital phase shift keying (PSK) or digital phase modulation, digital information is communicated over the channel by sequentially transmitting carrier pulses of constant amplitude, angular frequency, and duration, but of different relative phase. In biphase PSK, two signals having a 180.degree. relative phase difference between them are used to transmit one binary bit at a time. Demodulation of the PSK signal is accomplished by detecting the relative phase of the incoming signal and using the phase information thus obtained to determine the data content of the modulated signal.
Since different users of these communications channels often require different data rates, it is desirable to provide a PSK demodulator having sufficient flexibility to operate over a wide range of selectable data rates. Analog PSK demodulators currently in use generally require on-site calibration and periodic readjustment thereafter. These readjustment procedures are necessary to insure optimum performance of the demodulator since experience has shown that when analog units are not regularly adjusted, their operation tends to gradually deteriorate, often unnoticed, to the point where performance margins may be consumed and become unavailable when required.
Acquisition of the PSK signal in the first instance, and maintenance of signal lock following acquisition are also areas in which problems have previously been encountered.
The present invention overcomes these difficulties by providing a digital demodulator which can operate over a wide range of selected data rates without requiring either initial on-site adjustment or periodic readjustments thereafter. Further, the demodulator includes means for automatically acquiring the signal in the first instance, and for automatically reacquiring the signal should signal lock be broken. Several numerically controlled oscillators are also disclosed for use in this system.
In accordance with the present invention, a digital demodulator is disclosed which includes means for recovering a baseband signal from the modulated signal. The baseband signal is filtered through use of a matched filter to provide a soft data output. This output is multiplicatively combined with a normalization multiplier to provide a normalized data output. The normalization multiplier is derived by comparing the normalized data output with a selected threshold and increasing or decreasing the value of the multiplier in dependence upon the result of the comparison. Because of this normalization loop, the demodulator can accommodate a wide range of data rates without requiring extensive calibration.
In accordance with another aspect of the present invention, a carrier recovery loop is provided for a PSK demodulator which includes a lock detector for sensing when the signal has been acquired and when it has been lost. A carrier sweep control circuit responds to the output of the lock detector to cause a numerically controlled oscillator to sweep through a selected range of frequencies until the lock detector has sensed that the signal has been reacquired. The lock detector establishes this determination on the basis of several criteria: the amplitude of the data component of the incoming signal, indicated by the amplitude of the data normalization multiplier, must be greater than a threshold value; and the filtered and normalized in-phase component of the demodulated PSK signal must be greater by a selected amount than the filtered and normalized quadrature component of the demodulated PSK signal. A third criterion is established to insure that the demodulator does not falsely lock onto a bit-rate/2 frequency component.
In accordance with yet another aspect of the present invention, a numerically controlled oscillator is disclosed which includes accumulator means for accumulating control number at a substantially constant rate. The accumulator provides an overflow indication whenever the capacity of the accumulator is exceeded. Consequently, the rate at which overflow indications occur is dependent upon the magnitude of the control number supplied to the accumulator. An oscillator is thus produced whose frequency is numerically controlled.